Alcohols, in particular, 2-propanol (isopropyl alcohol-IPA), are being increasingly utilized in various industries as solvents and cleaning agents. Purification of alcohol streams when contaminated by water at certain concentrations becomes difficult due to the formation of azeotropic mixtures, composition wherein the ratios of the water and alcohol components in vapor and liquid are the same. Such azeotropic mixtures cannot be separated by normal distillation but only through complicated processes. Frequently, an additional substance is added to break down this azeotropic mixture. This additional substance must subsequently be completely removed and recovered from both product streams. Prior art processes using other membranes and gels did not provide the desirable properties of the processes of this invention.
The processes of the invention differ from microfiltration or ultrafiltration, processes wherein porosity is the key to preferential transport, and the flux rate depends upon molecular size. In the invention described herein using pervaporation/vapor permeation, molecular interaction between membrane and separated species is the determining factor rather than the molecular size. The main component of the pervaporation/vapor permeation process is the membrane material which determines the permeation and selectivity and hence the separation properties of the process.
A further criterion for the suitability of the per-vaporation membrane is its chemical and thermal stability. To obtain a high trans-membrane flow and a sufficient driving force, it is necessary to operate the pervaporation process at the highest possible temperatures. This means that the membrane will be in contact with a feed mixture containing a high concentration of organic components at high temperatures. To achieve an economical lifetime of the membranes all components of the membrane must be durable under aggressive conditions. The most common dehydration membrane reported in literature for use in pervaporation processes is prepared from polyvinyl alcohol (PVA).
Miyowasa (in U.S. Pat. Nos. 4,016,129 and 4,330,446) describes a hardenable coating comprising an aqueous dispersion of a silica polyvinyl alcohol complex prepared by the reaction between colloidal silica dispersion and polyvinyl-alcohol solution. However, this patent does not teach the casting solution of the invention for use in making membranes.
U.S. Pat. No. 4,148,689 to Hino, et. al. describes immobilization of microorganisms by hydrophilic complex gels by mixing a water-soluble polymer selected from PVA, gelatin and carboxymethylcellulose with tetraalkoxy-silane; hydrolyzing the resulting mixture by the addition of acid to form a homogenous complex for dispersing the microbial cells in the gelling mixture. This patent mentions that it is impossible to obtain the homogenous complex of the transparent gel when silica sol and silica gel were mixed with water soluble polymers. The problems addressed in this patent are not related to the process of the instant invention.
U.S. Pat. No. 4,279,752 describes uniform, porous PVA membranes. This process involves extracting the silica particles from the partially developed membrane. The reference is not related to use of a homogenous dispersion of silicon dioxide particles.